Supersonic research aircraft from El Segundo: The Skystreak and Skyrocket

The story of Charles E. "Chuck" Yeager's historic flight over the Mojave Desert in southern California on October 14, 1947 in the Bell X-1 supersonic research aircraft remains one of the most pivotal milestones in the history of not just post-World War II aviation, but also that of aviation development in southern California. However, the United States Air Force that sponsored development of the X-1 was not alone in encouraging the construction of experimental aircraft to investigate flight at transonic/supersonic speeds. Around the time that the Air Force commenced development of the Bell X-1, the US Navy launched their own program for a supersonic research aircraft in the late 1940s. During my visit to the Planes of Fame Museum in Chino, California, on April 13, 2019, I happened to come upon one of the three airframes of the Douglas D-558-2 Skyrocket supersonic rocket-powered research aircraft. Given that the US Navy's own supersonic research aircraft never received Navy designations, and they are at times overshadowed in the public eye by the X-1 and X-15, I though it would be very convenient to discuss in-depth the Navy's efforts in production research aircraft to investigate flight in the supersonic flight regime.

    Douglas D-558-1 Skystreak

In the mid-1940s, around the time that Bell was working on the XS-1 (later X-1) supersonic research aircraft, the US Navy and National Advisory Committee for Aeronautics (NACA) jointly initiated a research program to investigate the aerodynamic challenges of transonic and supersonic flight. This scheme, designated D-558 by Douglas, was originally to proceed in three phases: (1) a jet-powered transonic/supersonic research aircraft; (2) mixed rocket/jet-powered design; and (3) design of a combat plane. The first phase of this program was designated D-558-1 (christened Skystreak), and six aircraft were ordered by the Navy (to be fitted with nose and side air inlets and varying wing airfoil sections), while the second phase was called D-558-2 (christened Skyrocket). However, on January 27, 1947, the original D-558 blueprint was revised whereby the D-558-1 order was reduced to three aircraft (BuNos 37970/37972) with a single nose inlet, and the D-558-2 design conceived as a sleek, airplane with backswept wings. Three D-558-2 aircraft (BuNos 37973/37975) were ordered as substitutes for the final three D-558-1s that had been canceled. Construction of the first D-558-1 Skystreak began in 1946 and was finished in January 1947, with the first flight taking place on April 14. The Skystreak set a world speed air record of 641 miles per hour (1,031 km/h) on August 20, which was broken five days later by the second D-558-1, which had made its first flight that month. The design of the Skystreak featured a streamlined fuselage with a nose inlet (made from magnesium) and straight wings (made of aluminum), and power was supplied by Allison J35 turbojet. The first Skystreak made a total of 101 flights before being handed over to the NACA flight test unit at Edwards Air Force in April 1949. The second D-558-1 flew a total of 46 flights (27 by Douglas and the Navy, 19 by NACA) before it crashed on takeoff on May 3, 1948, due to compressor disintegration, killing test pilot Howard Lilly. The third D-558-3 began flying in 1949 and conducted a total of 81 flights until June 10, 1953, when it was retired by NACA. Like the X-1, the Skystreak was instrumental in providing a library of data on the fundamentals of transonic and near-supersonic flight, taking the US Navy closer to entering the supersonic age.

   

Top: Douglas D-558-2 Skyrocket (BuNo 39374) at the Planes of Fame Museum
Bottom: Third Douglas D-558-2 Skyrocket (BuNo 39375) at Edwards Air Force Base

Even while tests flights of the Skystreak were ongoing, the El Segundo Division of Douglas proceeded in earnest with development of the D-558-2 Skyrocket, which had a sleek, streamlined fuselage with a pointed nose and backswept wings and horizontal stabilizers. Like the Skystreak, the fuselage of the Skyrocket was made of magnesium and the wings and tail empennage were constructed from aluminum alloys. Although the Skyrocket was initially configured with a flush cockpit canopy, it was clear that visibility from this cockpit design was poor, so it was reconfigured with a raised cockpit with conventional angled windows. The first flight of the D-558-2 took place on February 4, 1948, with test pilot John F. Martin at the controls, and the first and second Skyrocket airframes initially flew using a single Westinghouse J34 turbojet only, but were later modified to include a Reaction Motors LR-8 four-chambered liquid-fuel rocket engine, while the third D-558-2 was completed with both the turbojet and rocket motor. A total of 313 flights were made by the Skyrocket (123 by BuNo 37973, 103 by BuNo 37974, and 87 by BuNo 37975), and the first Skyrocket was modified with air-launch capability and pure rocket power in 1955. One Boeing P2B-1S (patrol version of the B-29 Superfortress), christened Fertile Myrtle (BuNo 84029, previously USAAF serial number 45-21787), was modified to serve as the mothership for the Skyrocket, and the first aerial launch of the D-558-2 from the P2B-1S took place on September 8, 1950. On November 20, 1953 (nearly a month before the 50th anniversary of the Wright brothers' historic flight at Kitty Hawk in 1903), the Skyrocket became the first aircraft to reach Mach 2 when NACA pilot Scott Crossfield took the second D-558-2 into a dive to attain a top speed of Mach 2.005 (1,291 miles per hour, 2,078 km/h). The last flight of the Skyrocket occurred on December 20, 1956, and the three Skyrocket aircraft now reside in air museums, with BuNo 37973 (photographed by me in 2019) now displayed at the Planes of Fame Museum in Chino, California, BuNo 37974 (the only Skyrocket to reach Mach 2) on display at the National Air and Space Museum in Washington, D.C., and BuNo 37975 displayed on a pylon on the grounds of Antelope Valley College in Lancaster, California.

Left: Douglas Model 674; Right: Douglas Model 684

As an important footnote, in 1954, Douglas proposed a high-altitude hypersonic research aircraft, the Model 674, after the US Navy's Office of Naval Research awarded a preliminary contract for Douglas to look into a new-generation high-speed research aircraft. The Model 671 combined the straight wings of the Skystreak with the fuselage and tail empennage of the Skyrocket and was designed to reach an altitude of 1,000,000 feet and a speed of Mach 7. However, the performance benchmarks laid out for the Model 671 were rather too ambitious, and when the US Air Force, US Navy, and NACA announced a competition in December 1954 for a hypersonic research aircraft able to reach Mach 6.7 and 250,000 feet, Douglas unveiled a less ambitious but more radical proposal, the Model 684, which had stubby wings and the horizontal stabilizers and vertical fins arranged in a cruciform manner. The Model 684 was submitted in May 1955 along with the Republic AP-76, Bell D-171, and the North American design, and on September 30, North American's design was declared the winner and designated X-15. Some sources refer to the Model 671 and Model 684 projects as "D-558-3" or "Skyflash", but there is no evidence from Douglas company documents that either of these design was designated as much or given a company name.

Although overshadowed by the Bell X-1 in the public imagination, the Douglas D-558-1 Skystreak and D-558-2 Skyrocket played a role in helping the US Navy enter the supersonic age by providing a huge wealth of data on flight in the transonic and supersonic areas.